1. Field of the Invention
The present invention relates to a multi-carrier communication apparatus for use in a multi-carrier transmission system that is used to estimate the state of the channel using a reference signal.
2. Description of the Related Art
At present, the role of data transmission by radio is gaining more and more importance, due to the popularization of cellular phones, wireless LANs, and digital broadcast. In light of portability, ease of installation and cost, radio data transmission is significantly advantageous over wired data transmission. On the other hand, as radio is affected by the state of the airwaves, this affects the state of the channel used, thus also the communication quality. Communication can be seriously degraded, due to the phenomenon of multi-reflection radio propagation (multi-path).
Multi-carrier transmission, in which data is modulated into plural subcarriers and then transmitted, is known as a system for reducing the influence of multi-path. OFDM (Orthogonal Frequency Division Multiplexing) is one such transmission system. On the other hand, to cope with the changes in the channel state, a method of estimating the state of the channel has been developed. In this method, a known reference signal, called a pilot signal, is inserted into the data (data signal).
The above multi-carrier transmission and reference signal transmission are effective for removing the influence suffered in the transmitted data on the channel. However, if both transmissions are combined, an optimum reference signal may be different every subcarrier. In the prior art, dynamic and appropriate control of the reference signal is not executed for each subcarrier. Accordingly, the communication quality may significantly degrade. Also, the redundant reference signal is transmitted, resulting in increasing the redundancy of the transmission.